function animate(obj, target, callback){
    clearInterval(obj.timer);
    obj.timer = setInterval(function() {
        var step = (target - obj.offsetLeft) / 10;
        step = step < 0 ? Math.floor(step) : Math.ceil(step);
        if(obj.offsetLeft == target){
            clearInterval(obj.timer);
            /* if(callback){
                callback();
            } */
            callback && callback();
        }
        obj.style.left = obj.offsetLeft + step + 'px';
    },15);
}